Linkers in Computer Systems at Carnegie Mellon

The content discusses the importance of linkers in computer systems through a case study and examples from Carnegie Mellon University's Introduction to Computer Systems lecture. It covers the process of static linking, reasons behind using linkers, and the steps involved in linker operations such as symbol resolution and relocation.

• Linkers
• Computer Systems
• Carnegie Mellon
• Static Linking
• Symbol Resolution

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1. Carnegie Mellon Linking 15-213 / 18-213: Introduction to Computer Systems 15thLecture, 19 June 2014 Instructors: Greg Kesden 1

2. Carnegie Mellon Today Linking Case study: Library interpositioning 2

3. Carnegie Mellon Example C Program main.c swap.c int buf[2] = {1, 2}; int main() { swap(); return 0; } extern int buf[]; int *bufp0 = &buf[0]; static int *bufp1; void swap() { int temp; bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } 3

4. Carnegie Mellon Static Linking Programs are translated and linked using a compiler driver: unix> gcc -O2 -g -o p main.c swap.c unix> ./p main.c swap.c Source files Translators (cpp, cc1, as) Translators (cpp, cc1, as) Separately compiled relocatable object files main.o swap.o Linker (ld) Fully linked executable object file (contains code and data for all functions defined in main.c and swap.c) p 4

5. Carnegie Mellon Why Linkers? Reason 1: Modularity Program can be written as a collection of smaller source files, rather than one monolithic mass. Can build libraries of common functions (more on this later) e.g., Math library, standard C library 5

6. Carnegie Mellon Why Linkers? (cont) Reason 2: Efficiency Time: Separate compilation Change one source file, compile, and then relink. No need to recompile other source files. Space: Libraries Common functions can be aggregated into a single file... Yet executable files and running memory images contain only code for the functions they actually use. 6

7. Carnegie Mellon What Do Linkers Do? Step 1. Symbol resolution Programs define and reference symbols (variables and functions): void swap() { } /* define symbol swap */ swap(); /* reference symbol swap */ int *xp = &x; /* define symbol xp, reference x */ Symbol definitions are stored in object file (by compiler) in symbol table. Symbol table is an array of structs Each entry includes name, size, and location of symbol. Linker associates each symbol reference with exactly one symbol definition. 7

8. Carnegie Mellon What Do Linkers Do? (cont) Step 2. Relocation Merges separate code and data sections into single sections Relocates symbols from their relative locations in the .o files to their final absolute memory locations in the executable. Updates all references to these symbols to reflect their new positions. 8

9. Carnegie Mellon Three Kinds of Object Files (Modules) Relocatable object file (.o file) Contains code and data in a form that can be combined with other relocatable object files to form executable object file. Each .o file is produced from exactly one source (.c) file Executable object file (a.out file) Contains code and data in a form that can be copied directly into memory and then executed. Aside: a.out assembler output Shared object file (.so file) Special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run-time. Called Dynamic Link Libraries (DLLs) by Windows 9

10. Carnegie Mellon Executable and Linkable Format (ELF) Standard binary format for object files One unified format for Relocatable object files (.o), Executable object files (a.out) Shared object files (.so) Generic name: ELF binaries 10

11. Carnegie Mellon ELF Object File Format Elf header Word size, byte ordering, file type (.o, exec, .so), machine type, etc. 0 ELF header Segment header table Page size, virtual addresses memory segments (sections), segment sizes. Segment header table (required for executables) .text section .text section Code .rodata section .data section .rodata section Read only data: jump tables, ... .bss section .symtab section .data section Initialized global variables .rel.txt section .rel.data section .bss section Uninitialized global variables Block Started by Symbol Better Save Space Has section header but occupies no space .debug section Section header table 11

12. Carnegie Mellon ELF Object File Format (cont.) .symtab section Symbol table Procedure and static variable names Section names and locations 0 ELF header Segment header table (required for executables) .rel.text section Relocation info for .textsection Addresses of instructions that will need to be modified in the executable Instructions for modifying. .text section .rodata section .data section .bss section .rel.data section Relocation info for .datasection Addresses of pointer data that will need to be modified in the merged executable .symtab section .rel.txt section .rel.data section .debug section Info for symbolic debugging (gcc -g) .debug section Section header table Section header table Offsets and sizes of each section 12

13. Carnegie Mellon Linker Symbols Global symbols Symbols defined by module m that can be referenced by other modules. E.g.: non-static C functions and non-static global variables. External symbols Global symbols that are referenced by module m but defined by some other module. Local symbols Symbols that are defined and referenced exclusively by module m. E.g.: C functions and variables defined with the staticattribute. Local linker symbols are not local program variables 13

14. Carnegie Mellon Resolving Symbols Global External Local Global int buf[2] = {1, 2}; int main() { swap(); return 0; } extern int buf[]; int *bufp0 = &buf[0]; static int *bufp1; void swap() { int temp; Global main.c External bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } Linker knows nothing of temp swap.c 14

15. Carnegie Mellon Relocating Code and Data Relocatable Object Files Executable Object File .text .data 0 System code Headers System data System code main() .text main.o swap() .text main() .data More system code int buf[2]={1,2} System data .data int buf[2]={1,2} int *bufp0=&buf[0] int *bufp1 swap.o .text .data .bss swap() .bss .symtab .debug int *bufp0=&buf[0] static int *bufp1 Even though private to swap, requires allocation in .bss 15

16. Carnegie Mellon Relocation Info (main) main.c main.o int buf[2] = {1,2}; int main() { swap(); return 0; } 0000000 <main>: 0: 8d 4c 24 04 lea 0x4(%esp),%ecx 4: 83 e4 f0 and $0xfffffff0,%esp 7: ff 71 fc pushl 0xfffffffc(%ecx) a: 55 push %ebp b: 89 e5 mov %esp,%ebp d: 51 push %ecx e: 83 ec 04 sub $0x4,%esp 11: e8 fc ff ff ff call 12 <main+0x12> 12: R_386_PC32 swap 16: 83 c4 04 add $0x4,%esp 19: 31 c0 xor %eax,%eax 1b: 59 pop %ecx 1c: 5d pop %ebp 1d: 8d 61 fc lea 0xfffffffc(%ecx),%esp 20: c3 ret -4 Disassembly of section .data: 00000000 <buf>: 0: 01 00 00 00 02 00 00 00 Source: objdump r -d 16

17. Carnegie Mellon Relocation Info (swap, .text) swap.c swap.o extern int buf[]; int *bufp0 = &buf[0]; Disassembly of section .text: 00000000 <swap>: 0: 8b 15 00 00 00 00 mov 0x0,%edx 2: R_386_32 6: a1 04 00 00 00 mov 0x4,%eax 7: R_386_32 b: 55 push %ebp c: 89 e5 mov %esp,%ebp e: c7 05 00 00 00 00 04 movl $0x4,0x0 15: 00 00 00 10: R_386_32 14: R_386_32 18: 8b 08 mov (%eax),%ecx 1a: 89 10 mov %edx,(%eax) 1c: 5d pop %ebp 1d: 89 0d 04 00 00 00 mov %ecx,0x4 1f: R_386_32 23: c3 ret buf static int *bufp1; buf void swap() { int temp; .bss buf bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } buf 17

18. Carnegie Mellon Relocation Info (swap, .data) swap.c Disassembly of section .data: extern int buf[]; int *bufp0 = &buf[0]; static int *bufp1; 00000000 <bufp0>: 0: 00 00 00 00 0: R_386_32 buf void swap() { int temp; bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } 18

19. Carnegie Mellon Executable Before/After Relocation (.text) 0000000 <main>: . . . e: 83 ec 04 sub $0x4,%esp 11: e8 fc ff ff ff call 12 <main+0x12> 12: R_386_PC32 swap 16: 83 c4 04 add $0x4,%esp . . . 0x80483b0 + (-4) - 0x8048392 = 0x1a 0x8048396 + 0x1a = 0x80483b0 08048380 <main>: 8048380: 8048384: 8048387: 804838a: 804838b: 804838d: 804838e: 8048391: 8048396: 8048399: 804839b: 804839c: 804839d: 80483a0: 8d 4c 24 04 lea 0x4(%esp),%ecx 83 e4 f0 and $0xfffffff0,%esp ff 71 fc pushl 0xfffffffc(%ecx) 55 push %ebp 89 e5 mov %esp,%ebp 51 push %ecx 83 ec 04 sub $0x4,%esp e8 1a 00 00 00 call 80483b0 <swap> 83 c4 04 add $0x4,%esp 31 c0 xor %eax,%eax 59 pop %ecx 5d pop %ebp 8d 61 fc lea 0xfffffffc(%ecx),%esp c3 ret 19

20. Carnegie Mellon Executable Before/After Relocation (.text) . . . e: 83 ec 04 sub $0x4,%esp 11: e8 fc ff ff ff call 12 <main+0x12> 12: R_386_PC32 swap 16: 83 c4 04 add $0x4,%esp . . . 0000000 <main>: 0x80483b0 + (-4) - 0x8048392 = 0x1a 0x8048396 + 0x1a = 0x80483b0 Address of .text = 0x8048380 Offset of relocation entry = 0x12 refptr = 0x8048392 Address of swap = 0x80483b0 *refptr = -4 PC-relative resolved value = 0x80483b0 + -4 + 0x8048392 = 0x1a 20

21. Carnegie Mellon Executable Before/After Relocation (.text) 0000000 <main>: . . . e: 83 ec 04 sub $0x4,%esp 11: e8 fc ff ff ff call 12 <main+0x12> 12: R_386_PC32 swap 16: 83 c4 04 add $0x4,%esp . . . 0x80483b0 + (-4) - 0x8048392 = 0x1a 0x8048396 + 0x1a = 0x80483b0 08048380 <main>: 8048380: 8048384: 8048387: 804838a: 804838b: 804838d: 804838e: 8048391: 8048396: 8048399: 804839b: 804839c: 804839d: 80483a0: 8d 4c 24 04 lea 0x4(%esp),%ecx 83 e4 f0 and $0xfffffff0,%esp ff 71 fc pushl 0xfffffffc(%ecx) 55 push %ebp 89 e5 mov %esp,%ebp 51 push %ecx 83 ec 04 sub $0x4,%esp e8 1a 00 00 00 call 80483b0 <swap> 83 c4 04 add $0x4,%esp 31 c0 xor %eax,%eax 59 pop %ecx 5d pop %ebp 8d 61 fc lea 0xfffffffc(%ecx),%esp c3 ret 21

22. Carnegie Mellon 0: 8b 15 00 00 00 00 mov 0x0,%edx 2: R_386_32 6: a1 04 00 00 00 mov 0x4,%eax 7: R_386_32 ... e: c7 05 00 00 00 00 04 movl $0x4,0x0 15: 00 00 00 10: R_386_32 14: R_386_32 . . . 1d: 89 0d 04 00 00 00 mov %ecx,0x4 1f: R_386_32 23: c3 ret Before relocation buf buf .bss buf buf After relocation 080483b0 <swap>: 80483b0: 80483b6: 80483bb: 80483bc: 80483be: 80483c5: 80483c8: 80483ca: 80483cc: 80483cd: 80483d3: 8b 15 20 96 04 08 mov 0x8049620,%edx a1 24 96 04 08 mov 0x8049624,%eax 55 push %ebp 89 e5 mov %esp,%ebp c7 05 30 96 04 08 24 movl $0x8049624,0x8049630 96 04 08 8b 08 mov (%eax),%ecx 89 10 mov %edx,(%eax) 5d pop %ebp 89 0d 24 96 04 08 mov %ecx,0x8049624 c3 ret 22

23. Carnegie Mellon Executable After Relocation (.data) Disassembly of section .data: 08049620 <buf>: 8049620: 01 00 00 00 02 00 00 00 08049628 <bufp0>: 8049628: 20 96 04 08 23

24. Carnegie Mellon Strong and Weak Symbols Program symbols are either strong or weak Strong: procedures and initialized globals Weak: uninitialized globals p1.c p2.c int foo=5; int foo; weak strong p1() { } p2() { } strong strong 24

25. Carnegie Mellon Linker s Symbol Rules Rule 1: Multiple strong symbols are not allowed Each item can be defined only once Otherwise: Linker error Rule 2: Given a strong symbol and multiple weak symbol, choose the strong symbol References to the weak symbol resolve to the strong symbol Rule 3: If there are multiple weak symbols, pick an arbitrary one Can override this with gcc fno-common 25

26. Carnegie Mellon Linker Puzzles int x; p1() {} Link time error: two strong symbols (p1) p1() {} int x; p1() {} int x; p2() {} References to x will refer to the same uninitialized int. Is this what you really want? int x; int y; p1() {} double x; p2() {} Writes to x in p2 might overwrite y! Evil! int x=7; int y=5; p1() {} double x; p2() {} Writes to x in p2will overwrite y! Nasty! References to x will refer to the same initialized variable. int x=7; p1() {} int x; p2() {} Nightmare scenario: two identical weak structs, compiled by different compilers with different alignment rules. 26

27. Carnegie Mellon Role of .h Files c1.c global.h #ifdef INITIALIZE int g = 23; static int init = 1; #else extern int g; static int init = 0; #endif #include "global.h" int f() { return g+1; } c2.c #include <stdio.h> #include "global.h" int main() { if (init) // do something, e.g., g=31; int t = f(); printf("Calling f yields %d\n", t); return 0; } 27

28. Carnegie Mellon Running Preprocessor c1.c global.h #ifdef INITIALIZE int g = 23; static int init = 1; #else extern int g; static int init = 0; #endif #include "global.h" int f() { return g+1; } -DINITIALIZE no initialization int g = 23; static int init = 1; int f() { return g+1; } extern int g; static int init = 0; int f() { return g+1; } #include causes C preprocessor to insert file verbatim (Use gcc E to view result) 28

29. Carnegie Mellon Role of .h Files c1.c global.h #ifdef INITIALIZE int g = 23; static int init = 1; #else extern int g; static int init = 0; #endif extern int g; static int init = 0; #include "global.h" int f() { return g+1; } c2.c #define INITIALIZE #include <stdio.h> #include "global.h" int g = 23; static int init = 1; int main() { if (init) // do something, e.g., g=31; int t = f(); printf("Calling f yields %d\n", t); return 0; } 29

30. Carnegie Mellon Global Variables Avoid if you can Otherwise Use static if you can Initialize if you define a global variable Use extern if you use external global variable 30

31. Carnegie Mellon Packaging Commonly Used Functions How to package functions commonly used by programmers? Math, I/O, memory management, string manipulation, etc. Awkward, given the linker framework so far: Option 1: Put all functions into a single source file Programmers link big object file into their programs Space and time inefficient Option 2: Put each function in a separate source file Programmers explicitly link appropriate binaries into their programs More efficient, but burdensome on the programmer 31

32. Carnegie Mellon Solution: Static Libraries Static libraries (.a archive files) Concatenate related relocatable object files into a single file with an index (called an archive). Enhance linker so that it tries to resolve unresolved external references by looking for the symbols in one or more archives. If an archive member file resolves reference, link it into the executable. 32

33. Carnegie Mellon Creating Static Libraries atoi.c printf.c random.c ... Translator Translator Translator atoi.o printf.o random.o unix> ar rs libc.a \ atoi.o printf.o random.o Archiver (ar) libc.a C standard library Archiver allows incremental updates Recompile function that changes and replace .o file in archive. 33

34. Carnegie Mellon Commonly Used Libraries libc.a (the C standard library) 8 MB archive of 1392 object files. I/O, memory allocation, signal handling, string handling, data and time, random numbers, integer math libm.a (the C math library) 1 MB archive of 401 object files. floating point math (sin, cos, tan, log, exp, sqrt, ) % ar -t /usr/lib/libc.a | sort fork.o fprintf.o fpu_control.o fputc.o freopen.o fscanf.o fseek.o fstab.o % ar -t /usr/lib/libm.a | sort e_acos.o e_acosf.o e_acosh.o e_acoshf.o e_acoshl.o e_acosl.o e_asin.o e_asinf.o e_asinl.o 34

35. Carnegie Mellon Linking with Static Libraries multvec.o addvec.o main2.c vector.h Archiver (ar) Translators (cpp, cc1, as) Static libraries libvector.a libc.a printf.o and any other modules called by printf.o Relocatable object files main2.o addvec.o Linker (ld) Fully linked executable object file p2 35

36. Carnegie Mellon Using Static Libraries Linker s algorithm for resolving external references: Scan .o files and .a files in the command line order. During the scan, keep a list of the current unresolved references. As each new .o or .a file, obj, is encountered, try to resolve each unresolved reference in the list against the symbols defined in obj. If any entries in the unresolved list at end of scan, then error. Problem: Command line order matters! Moral: put libraries at the end of the command line. unix> gcc -L. libtest.o -lmine unix> gcc -L. -lmine libtest.o libtest.o: In function `main': libtest.o(.text+0x4): undefined reference to `libfun' 36

37. Carnegie Mellon Loading Executable Object Files Memory outside 32-bit address space Executable Object File Kernel virtual memory 0 0x100000000 ELF header User stack (created at runtime) Program header table (required for executables) %esp (stack pointer) .init section .text section Memory-mapped region for shared libraries .rodata section 0xf7e9ddc0 .data section .bss section brk Run-time heap (created by malloc) .symtab .debug Loaded from the executable file Read/write segment (.data, .bss) .line .strtab Read-only segment (.init, .text, .rodata) Section header table (required for relocatables) 0x08048000 Unused 0 37

38. Carnegie Mellon Shared Libraries Static libraries have the following disadvantages: Duplication in the stored executables (every function need std libc) Duplication in the running executables Minor bug fixes of system libraries require each application to explicitly relink Modern solution: Shared Libraries Object files that contain code and data that are loaded and linked into an application dynamically, at either load-time or run-time Also called: dynamic link libraries, DLLs, .so files 38

39. Carnegie Mellon Shared Libraries (cont.) Dynamic linking can occur when executable is first loaded and run (load-time linking). Common case for Linux, handled automatically by the dynamic linker (ld-linux.so). Standard C library (libc.so) usually dynamically linked. Dynamic linking can also occur after program has begun (run-time linking). In Linux, this is done by calls to the dlopen() interface. Distributing software. High-performance web servers. Runtime library interpositioning. Shared library routines can be shared by multiple processes. More on this when we learn about virtual memory 39

40. Carnegie Mellon Dynamic Linking at Load-time main2.c vector.h unix> gcc -shared -o libvector.so \ addvec.c multvec.c Translators (cpp, cc1, as) libc.so libvector.so Relocatable object file main2.o Relocation and symbol table info Linker (ld) Partially linked executable object file p2 Loader (execve) libc.so libvector.so Code and data Fully linked executable in memory Dynamic linker (ld-linux.so) 40

41. Carnegie Mellon Dynamic Linking at Run-time #include <stdio.h> #include <dlfcn.h> int x[2] = {1, 2}; int y[2] = {3, 4}; int z[2]; int main() { void *handle; void (*addvec)(int *, int *, int *, int); char *error; /* Dynamically load the shared lib that contains addvec() */ handle = dlopen("./libvector.so", RTLD_LAZY); if (!handle) { fprintf(stderr, "%s\n", dlerror()); exit(1); } 41

42. Carnegie Mellon Dynamic Linking at Run-time ... /* Get a pointer to the addvec() function we just loaded */ addvec = dlsym(handle, "addvec"); if ((error = dlerror()) != NULL) { fprintf(stderr, "%s\n", error); exit(1); } /* Now we can call addvec() just like any other function */ addvec(x, y, z, 2); printf("z = [%d %d]\n", z[0], z[1]); /* unload the shared library */ if (dlclose(handle) < 0) { fprintf(stderr, "%s\n", dlerror()); exit(1); } return 0; } 42

43. Carnegie Mellon Today Linking Case study: Library interpositioning 43

44. Carnegie Mellon Case Study: Library Interpositioning Library interpositioning : powerful linking technique that allows programmers to intercept calls to arbitrary functions Interpositioning can occur at: Compile time: When the source code is compiled Link time: When the relocatable object files are statically linked to form an executable object file Load/run time: When an executable object file is loaded into memory, dynamically linked, and then executed. 44

45. Carnegie Mellon Some Interpositioning Applications Security Confinement (sandboxing) Interpose calls to libc functions. Behind the scenes encryption Automatically encrypt otherwise unencrypted network connections. Monitoring and Profiling Count number of calls to functions Characterize call sites and arguments to functions Malloc tracing Detecting memory leaks Generating address traces 45

46. Carnegie Mellon Example program #include <stdio.h> #include <stdlib.h> #include <malloc.h> Goal: trace the addresses and sizes of the allocated and freed blocks, without modifying the source code. int main() { free(malloc(10)); printf("hello, world\n"); exit(0); } Three solutions: interpose on the libmalloc and free functions at compile time, link time, and load/run time. hello.c 46

47. Carnegie Mellon Compile-time Interpositioning #ifdef COMPILETIME /* Compile-time interposition of malloc and free using C * preprocessor. A local malloc.h file defines malloc (free) * as wrappers mymalloc (myfree) respectively. */ #include <stdio.h> #include <malloc.h> /* * mymalloc - malloc wrapper function */ void *mymalloc(size_t size, char *file, int line) { void *ptr = malloc(size); printf("%s:%d: malloc(%d)=%p\n", file, line, (int)size, ptr); return ptr; } mymalloc.c 47

48. Carnegie Mellon Compile-time Interpositioning #define malloc(size) mymalloc(size, __FILE__, __LINE__ ) #define free(ptr) myfree(ptr, __FILE__, __LINE__ ) void *mymalloc(size_t size, char *file, int line); void myfree(void *ptr, char *file, int line); malloc.h linux> make helloc gcc -O2 -Wall -DCOMPILETIME -c mymalloc.c gcc -O2 -Wall -I. -o helloc hello.c mymalloc.o linux> make runc ./helloc hello.c:7: malloc(10)=0x501010 hello.c:7: free(0x501010) hello, world 48

49. Carnegie Mellon Link-time Interpositioning #ifdef LINKTIME /* Link-time interposition of malloc and free using the static linker's (ld) "--wrap symbol" flag. */ #include <stdio.h> void *__real_malloc(size_t size); void __real_free(void *ptr); /* * __wrap_malloc - malloc wrapper function */ void *__wrap_malloc(size_t size) { void *ptr = __real_malloc(size); printf("malloc(%d) = %p\n", (int)size, ptr); return ptr; } mymalloc.c 49

50. Carnegie Mellon Link-time Interpositioning linux> make hellol gcc -O2 -Wall -DLINKTIME -c mymalloc.c gcc -O2 -Wall -Wl,--wrap,malloc -Wl,--wrap,free \ -o hellol hello.c mymalloc.o linux> make runl ./hellol malloc(10) = 0x501010 free(0x501010) hello, world The -Wl flag passes argument to linker Telling linker --wrap,malloc tells it to resolve references in a special way: Refs to malloc should be resolved as __wrap_malloc Refs to __real_malloc should be resolved as malloc 50

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